Make sure you’re pushing your Nerf darts completely into the chambers, and they’re not loose. If the darts can't be pushed in completely, then make sure there are no item(s) or missing gum obstructing the chambers. If there is, simply remove and try again.

Before delving into the guts of your Nerf gun, make sure the cocking device is working. Pull until you can't pull any further without dislodging the slide. If the cocking mechanism is not locking, then don't worry. Read below.

Wear and tear may impede the cocking mechanisms ability to lock in-place once pulled back the appropriate amount ¶

The spring that allows your cocking mechanism to cock might be broken, so follow the instructions in this repair guide. The Nerf gun most likely got hit on a hard surface.

Parts inside the gun may interfere with the trigger motion as well as missing or dislodged pieces ¶

If the trigger is jammed, fear not about replacing it; nobody sells it. Instead, it could be a broken spring that allows for the movement of the trigger. Any spring of similar size can be used as a substitute. If you open up the nerf gun and find the spring isn't broken, then follow this repair guide to see how to fix it back into place. Your sibling might have thought this was a cool looking bat, and decided to swing at some balls with it.

Likely cause if your brother got a few good hits using the Nerf gun as a bat. Worry not. If you can't see any observable damage on the gun, then you might want to open it up to check for any damage on the inside. Simply follow this repair guide.

Resources

Repairability

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